The present invention relates to a method for manufacturing laminated-ceramic electronic components such as laminated-ceramic capacitors which are widely used in electronic devices such as video tape recorders, liquid crystal display televisions and OA equipment. The invention can also be widely applied to manufacturing of laminated-ceramic electronic components such as multilayer ceramic substrates, laminated varistors and laminated piezoelectric devices.
In recent years, the trend toward higher density of circuit boards in the field of electronic components has been imposing increasing requirements on laminated-ceramic capacitors and other components for further size reduction and higher performance. A laminated-ceramic capacitor is taken as an example in the following description.
FIG. 1 is a cross sectional view of a part of a laminated-ceramic capacitor. In FIG. 1, the reference numeral 1 is the ceramic layer, 2 is internal electrodes and 3 is external electrodes. The internal electrodes 2 are connected alternately to the two external electrodes 3.
Laminated-ceramic capacitors have been manufactured by such a conventional method as follows.
Specified electrode ink is printed on ceramic greenware sheets which have been cut into a specified size. The electrode ink is dried to form an electrode ink film. Then a specified number of ceramic greenware sheets on which the electrode ink films have been formed are laminated to form a laminated body of ceramic greenware sheets which is then cut into a desired shape and sintered, to which external electrodes are then bonded for completion.
However, a direct printing of the electrode ink on the ceramic greenware sheet has drawbacks such that a solvent included in the electrode ink (solvents such as diethylene glycol monobutyl ether of about 30% by weight is included in the electrode ink available in the market) causes the ceramic greenware sheet to swell and/or corrodes it during the printing of the electrode ink onto the ceramic greenware sheet. In addition, pin holes become more likely to occur in the ceramic greenware sheet as the ceramic greenware sheet becomes thinner, resulting in a short circuit between internal electrodes.
Various approaches have been taken in an attempt to solve these problems.
FIG. 2 is a drawing explanatory of a process for printing electrode ink on the ceramic greenware sheet by means of a screen printing technique. In FIG. 2, the reference numeral 4 is the screen frame, 5 is the screen, 6 is electrode ink, 7 is the squeegee, 8 is the stage, 9 in the base film, 10 is the ceramic greenware sheet and 11 is the printed electrode. The arrow indicates the direction in which the squeegee moves. As shown in FIG. 2, a specified number of ceramic greenware sheets on which the electrode ink is printed are laminated to form a ceramic greenware lamination, which is then cut into a desired shape and then sintered, to which external electrodes are bond ed to make laminated-ceramic capacitors.
In order to increase the capacitance of the laminated-ceramic capacitor, it has been attempted to decrease the thickness of the ceramic layer. In order to decrease the thickness of the ceramic layer, thickness of the ceramic greenware sheet must be decreased. However, a decrease in the thickness of the ceramic greenware sheet leads to the decrease of its mechanical strength. Therefore, there is a problem that a ceramic greenware sheet of a thickness less than 20 micrometers cannot be handled individually (called problem 1 hereafter), which makes a reduction of thickness difficult. Moreover, during a printing of electrode ink onto the ceramic greenware sheet surface, there is a problem that the likeliness of a short circuit occurrence increases with a decrease in the thickness of the ceramic greenware sheet (called problem 2 hereafter), because the ceramic greenware sheet swells or is dissolved by the electrode ink (or by permeation of the electrode ink into minute surface irregularities of the ceramic greenware sheet).
Also, with a laminated-ceramic capacitor, an increase in the number of laminated layers causes local thickness variations or steps on the surface due to the internal electrodes. This problem will be well understood from the following description with reference to FIGS. 3 and 4.
FIG. 3 is a cross sectional view of a laminated-ceramic capacitor formed by multiple layers. As shown in FIG. 3, the thickness of the laminated-ceramic capacitor is greater at the center (where the number of internal electrodes 2 stacked is greater), called thickness A, than at the periphery (where the number of internal electrodes 2 stacked is smaller), called thickness B.
FIG. 4 is a drawing explanatory of the thickness difference between the center and the periphery with respect to the number of laminated layers. The thickness of the ceramic greenware sheet used herein is 20 micrometers, and the thickness of the internal electrode 2 is 4 micrometers. From FIG. 4, it can be seen that a thickness difference between the center and the periphery exceeds 20 micrometers, which is equal to the thickness of the ceramic greenware sheet which is laminated, when the number of laminated layers exceeds 10. Surface irregularities due to this thickness difference (total thickness of electrodes) make it impossible to make uniformly laminated-ceramic capacitors, causing a problem such as delamination (peel-off of layers) and crack (called problem 3 hereafter).
To solve these problems (problems 1, 2 and 3), various approaches have been taken.
Solutions to the problem 1 include a method, proposed in Japanese Laid-Open Patent Publication No. 62-63413, where electrode ink is printed onto the ceramic greenware sheets while the ceramic greenware sheets remain to be adhered onto base films, and they are laminated to make it easy to handle the ceramic greenware sheets. After lamination, the base films are peeled off. This method can solve problem 1 to a certain extent, but cannot solve problem 3. Moreover, with regards to problem 2, the solvent of the electrode ink which is absorbed in one side of the ceramic is greenware sheet cannot evaporate from the opposite side, because the opposite side (on which electrode ink is not printed) of the ceramic greenware sheet is covered by the base film, and remains in the ceramic greenware sheet. Consequently, the electrode ink remains in the ceramic greenware sheet for a longer period of time than in the case of the conventional method, thereby further aggravating the problem that the ceramic greenware sheet becomes susceptible to corrosion by the electrode ink (problem 2).
Another method proposed in Japanese Laid-Open Patent Publication No. 59-172711 is to manufacture laminated-ceramic capacitors by embedding electrodes, which are formed on a base film, in ceramic greenware sheets, laminating the ceramic greenware sheets together with the base films, and sintering them. Although this method solves problems 2 and 3 to a certain extent, problem 1 remains unsolved. This is because the base film must be very thin, namely from 1.5 to 14 micrometers, in order to sinter the laminated-ceramic greenware sheets together with the base films. Moreover, the number of base films to be sintered increases with an increase in the number of laminated layers, which makes delamination more likely to occur. This implies that the base film thickness must be decreased with an increase in the number of laminated layers. Base films of such a small thickness are difficult to handle, weak in mechanical strength and are not practical.
Approaches to solve problem 2 include one which uses transfer of electrodes. In Japanese Laid-Open Patent Publication No. 52-15879, Japanese Laid-Open Patent Publication No. 63-31104 and Japanese Laid-Open Patent Publication No. 63-32909, such methods are proposed that electrode ink films are formed on ceramic greenware sheets while adverse effects of the solvent included in the electrode ink are being prevented by subjecting the ceramic greenware sheet to a thermal transfer process with electrode ink, instead of a printing process, thereby improving a yield of laminated-ceramic capacitors. However, in these methods, ceramic greenware sheets and electrodes are laminated alternately by thermal transfer. This means that, to laminate internal electrodes in 50 layers, at least 100 times of thermal transfer are repeated in all, 50 times or more for lamination of ceramic greenware sheets and 50 times for lamination of electrodes. Also, because the likeliness of an occurrence of pin holes is high with only one layer of ceramic greenware sheet, consecutive transfer of two ceramic greenware sheets may be tried, which results in the number of thermal transfer cycles totaling to 150 or more. Moreover, the heating cycles which one transferred layer has undergone is different from those which another layer has undergone. That is, the layer which was laminated first undergoes about 150 heating cycles which follow, while the last laminated layer undergoes no subsequent heat cycles. In general, ceramic greenware sheets undergo slight thermal deformations every time such a heat cycle is applied. As a result, layers which were thermally transferred earlier undergo more heat cycles and therefore undergo greater thermal deformation than the layers laminated later. This results in deformation or a thickness change of the ceramic greenware sheets, a change in the electrode surface area or a disagreement of the position of laminated electrodes, likely causing an occurrence of defects during a cutting-off process after lamination. Moreover, this requires the electrodes to be transferred later on the ceramic greenware sheets which have been formed in a uniform thickness, and consequently it cannot be avoided that a lamination unevenness occurs due to the thickness of the electrodes, leaving problem 3 unsolved. Also in Japanese Laid-Open Patent Publication No. 63-51616, such a method is proposed that an electrode ink film is placed over a ceramic greenware sheet after forming the electrode ink film on a base film, and the electrode ink film is thermally transferred onto the ceramic greenware sheet, next of which a plurality of ceramic greenware sheets are laminated and sintered. With this method, however, thermal transfer of electrode ink film is needed before the lamination of the ceramic greenware sheets by means of heat, and additional heat cycles are applied to the ceramic greenware sheet, resulting in poor accuracy. It also causes the electrode to be thermally transferred after thermal transfer onto a ceramic greenware sheet which has thermally softening characteristics, not onto a base film of accurate size. During this thermal transfer of the electrode, not only the ceramic greenware sheet but also the base film undergo thermal deformation, which causes an increase in the possibility of deterioration in the lamination accuracy. Therefore, it is indispensable in this method to use base films of excellent heat resistance (free from thermal deformation), which causes an increase in the production cost. Moreover, electrodes are transferred later on the surfaces of the ceramic greenware sheets which have been formed with uniform thickness in this method, which causes difficulties to avoid lamination irregularities that occur due to the thickness of the electrodes, leaving the problem 3 unsolved.
Also, in Japanese Laid-Open Patent Publication No. 63-51617, a method of electrode transfer is proposed in which a given pattern of electrodes is transferred from an electrode layer onto a ceramic greenware sheet by heat-pressing a film against the ceramic greenware sheet by means of a die which has projections corresponding to the electrode pattern. In this case, however, the projection of the die unavoidably causes the ceramic greenware sheet thickness to change at points where it is pressed by the projections. Moreover, the same number of projections as the electrodes are needed, resulting in a variation of the pressure from projection to projection. This also results in a variation of the thickness or compression ratio of the portion of the ceramic greenware sheet corresponding to the position of each electrode. Moreover, even in a projection for the transfer of a single electrode, there exists a pressure distribution (a phenomenon generally called the marginal zone which causes an unevenness in the density of the printed ink in a relief press process) which causes the thickness of the ceramic greenware sheet to vary. Furthermore, because the base film, on which the ceramic greenware sheet is formed, as well as the ceramic greenware sheet are subject to a local thermal pressure before lamination, they are likely to undergo irregular deformation. Also, in the same way as that of the method proposed in Japanese Laid-Open Patent Publication No. 63-51616, thermal transfer of the electrode is performed not on a base film of an accurate size but is performed after thermal transfer onto a ceramic greenware sheet which has thermal softening characteristics. Thus, this method cannot solve problem 3, as described in the method of Japanese Laid-Open Patent Publication No. 63-51616.
In Japanese Laid-Open Patent Publication No. 63-53912, a method for forming internal electrodes for ceramic lamination is proposed where an electrode ink film which includes UV setting resins to form the internal electrodes is transferred onto a ceramic greenware sheet and then laminated. With this method, however, the part of the electrode ink which is in contact with the carrier film hardens, and the part of the electrode ink positioned on its surface (opposite to the carrier film) does not undergo complete hardening or does not harden at all and remains sticky. The surface of the electrode ink that is in a half-dried condition as mentioned above tends to catch dust and dirt if the chance allows, and is difficult to be handled. After printing the electrode ink on the carrier film, the film cannot be wound because the surface of the film is not dried. Moreover, after transfer of the electrode ink onto the ceramic greenware sheet, the ceramic greenware sheet is not held on the carrier film but is laminated. Therefore, when the ceramic greenware sheet has a thickness of 20 micrometers or less, it is too weak in mechanical strength to be handled. As a result, there is a limitation to the decrease in the thickness of ceramic greenware sheets.
Approaches to solve problem 2 include one which forms electrodes on a base film beforehand. For example, Japanese Patent Publication No. 40-19975 proposes a method where an electrode ink film is printed on a base film, and then a ceramic greenware sheet is formed thereon by a casting process. Japanese Patent Publication No. 40-19975 proposes a method where ceramic greenware sheets with electrodes embedded therein are obtained by applying electrode ink to base films and drying it, then continuously applying dielectric material slurry thereto, and peeling it off from the support (base film). Although this-method solves problem 2 to some extent, it cannot solve problem 1. That is, because ceramic greenware sheets with electrodes embedded therein prepared by those methods are laminated after being peeled off from the base film, the mechanical strength is drastically decreased with a decrease in the film thickness. Thus, the problem that the sheets cannot be handled when the thickness thereof is 20 micrometers or less (problem 1) remains unsolved.
Methods for manufacturing laminated-ceramic capacitors by embedding electrodes in ceramic greenware sheets are proposed in Japanese Laid-Open Patent Publication No. 55-124225 and Japanese Patent Publication No. 62-35255. these methods are to laminate a dielectric material and an electrode alternately on a single base film so as to form multiple layers by means of a gravure printing or the like, and are not free from the problem of the ceramic greenware sheets being eroded by the solvent included in the electrode.
Moreover, Japanese Patent Publication No. 60-49590 proposes a method in which a first electrode is printed onto a first ceramic greenware sheet, then a paste which includes ceramic powder is printed so that it covers the first electrode and is dried to form a first auxiliary ceramic layer, and a second ceramic greenware sheet is laminated onto it. With this method, a ceramic greenware sheet layer and an auxiliary ceramic layer are formed between the internal electrodes; namely, double ceramic greenware sheet layers are formed. A similar method is proposed in Japanese Patent Publication No. 62-27721. However, with either method, the ceramic greenware sheet on which an electrode is formed (or an electrode is embedded therein) is laminated in a state of being peeled off from the base film. As a result, when the thickness of the ceramic greenware sheet is decreased below 20 micrometers, the ceramic greenware sheet becomes too weak in mechanical strength to be handled. Thus there is a limit to a decrease in the ceramic greenware sheet thickness.
Japanese Laid-Open Patent Publication Nos. 52-135050 and 52-133553 propose such methods that a ceramic greenware sheet from which portions corresponding to the internal electrodes at the part of a step, i.e., the peripheral section, have been removed is interposed, laminated and sintered. These methods require to accurately remove several hundreds of portions with a size of e.g., measuring 3.5 mm by 1.0 mm each from the ceramic greenware sheet. However, it is almost impossible to handle the ceramic greenware sheet individually because of its thinness and softness. Even though it can be handled, it is very difficult to punch it off precisely.
Similarly in Japanese Laid-Open Patent Publication No. 61-102719, a method for manufacturing laminated-ceramic capacitors by punching both ceramic greenware sheets and electrode sheets and laminating them alternately is proposed. However, this has a problem in the applicability to mass production. For example, it is disadvantageous to punch many pieces at a time compared to a printing process, in terms of the production cost and accuracy. In addition, when the thickness of electrodes is decreased below 5 micrometers, physical strength of the electrode sheet cannot withstand the punching and handling processes.
Japanese Laid-Open Patent Publication No. 52-135051 proposes a method in which an internal electrode ink is first applied to a ceramic greenware sheet, and a dielectric ink is applied to the rest of the ceramic greenware sheet surface, and ceramic greenware sheets with the electrode ink and the dielectric ink applied thereto are laminated while shifting the position to avoid overlapping of leads of the electrodes, and are pressurized and sintered. However, this method leaves a problem that lower layers in the lamination are eroded by solvent included in the dielectric ink which is to be printed later. As a result, the thinner the ceramic greenware sheet, the more likely the characteristics of short circuit and withstanding voltage to deteriorate.
A method for forming electrodes without using any solvent in electrodes ink is proposed in Japanese Laid-Open Patent Publication No. 53-51458. There is also a method based on a non-electrolytic plating technique using an activated paste, such as that which is proposed in Japanese Laid-Open Patent Publication No. 57-102166. However, since the electrodes are formed by means of a non-electrolytic plating technique, immersion of ceramic greenware sheets in a plating liquid causes a new problem.
Other methods of partially punching ceramic greenware sheets or embossing them have been proposed such as those in Japanese Laid-Open Patent Publication No. 53-42353, but they are not practical.
A method proposed in Japanese Laid-Open Patent Publication No. 56-94719 is similar to the above, and is to form ceramic greenware sheets at steps which have arisen on the laminated layers. This method is not suitable for mass production,
Problems which make it difficult to manufacture laminated-ceramic capacitors such as follows have been described in the above, explaining that various methods proposed so far are not capable of satisfactorily solve problems 1 through 3: Insufficient mechanical strength due to the thinness of ceramic greenware sheets and an accompanying difficulty in handling them (problem 1), an increase in the probability of a short circuit occurrence due to the effect of the ink during a printing process of the electrode ink on the ceramic greenware sheet as the ceramic greenware sheet is made thinner (problem 2), and defects such as delamination and crack due to an unevenness caused by the difference in the thickness between the center and the periphery, which makes uniform lamination impossible (problem 3).
In addition to the problems described above, a conventional method includes such a problem that wire breakage or lower product quality is caused when electrodes are impaired during the preparation of ceramic greenware sheets with electrodes embedded therein by applying ceramic slurry in a thin film on the electrodes formed on a support.
Thus, there have been no proper method for applying ceramic slurry by which electrodes are embedded therein. This situation will be explained in the following with reference to FIGS. 5 and 6.
FIG. 5 is a drawing explanatory of a process for applying ceramic slurry by the use of a gravure printing press. In FIG. 5, the reference numeral 12 is ceramic slurry, 13 is the gravure plate, 14 is the cell, 15 is the doctor and 16 is the impression cylinder. The ceramic slurry 12 enters into the cell 14 which is formed on the surface of the gravure plate 13 and is, after the excess ceramic slurry is scraped off by the doctor 15, applied onto the surface of a base film 9 which is pressed against the gravure plate 13 by the impression cylinder 16. Then the ceramic slurry 12 applied onto the surface of the base film 9 is dried, resulting in a ceramic greenware sheet 10.
FIG. 6 is a drawing explanatory of a process for applying ceramic slurry onto a base film with an electrode formed thereon by means of a gravure printing press. In FIG. 6, the reference numeral 17 is the electrode which is dried after printing. What is different from FIG. 5 is that the electrode 17 is formed on the surface of the base film 9 in advance. In a case that the electrode 17 is embedded in the ceramic greenware sheet 10 as shown in FIG. 6, the pressure of the gravure printing press to apply the ceramic slurry cannot be decreased because of the presence of the impression cylinder 16, resulting in damage to the electrode 17 and making it impossible to stably embed the electrode in the ceramic greenware sheet 10.
FIGS. 7(A) to (C) are drawings explanatory of irregularities, due to the electrodes, being generated on the slurry surface as the ceramic slurry is dried after embedding the electrodes in the ceramic slurry. In FIG. 7, the reference numeral 18 is the ceramic greenware sheet on which surface irregularities have been generated and 19 is the ceramic greenware sheet with electrodes embedded therein where surface irregularities have been generated.
The electrode 17 which is formed on the base film 9 as shown in FIG. 7(A) is embedded in the ceramic slurry 12 as shown in FIG. 7(B). Then, when the ceramic slurry 12 is dried to form the ceramic greenware sheet 18, surface irregularities due to the electrode 17 are generated on the ceramic greenware sheet 18 as shown in FIG. 7(C). While the generation of such surface irregularities depends on the thickness of the electrode 17, it becomes conspicuous as the thickness of the ceramic greenware sheet 18 is decreased, below 20 micrometers.
The present invention, which solves the problems with the prior art, is to provide a method for manufacturing laminated-ceramic electronic components which does not deteriorate the handleability of ceramic greenware sheets even when their thickness is decreased, and is capable of reducing the occurrence of surface irregularities due to the thickness of the electrodes even when the number of layers laminated is increased.
In order to achieve the above-mentioned object, the method for manufacturing laminated-ceramic electronic components according to the invention is characterized in that a ceramic greenware sheet with electrodes embedded therein which is formed on a support is, without being peeled off from the support, pressure-bonded on a second ceramic greenware sheet or other electrodes, then the support alone is peeled off and the ceramic greenware sheet with the embedded electrodes is transferred onto the second ceramic greenware sheet or the other electrodes to complete a lamination.
This manufacturing method does laminate thin ceramic greenware sheets, the thickness of each of which is as thin as below 20 micrometers while maintaining their mechanical strength by handling electrodes and ceramic greenware sheets together with supports.
Also, according to the invention, because the electrodes are embedded in the ceramic greenware sheets, an occurrence of surface irregularities due to the electrodes can be reduced more than in the case that electrodes on the ceramic greenware sheets are formed.
Especially, according to the invention, an occurrence of irregularities on the surface of a ceramic greenware sheet with electrodes embedded therein due to the electrodes, can be prevented and a ceramic greenware sheet with electrodes embedded therein, the smoothness of the surface of which is effectively achieved, can be manufactured by methods such as; exposing a ceramic slurry, which has been applied to a support, to air blow in a direction other than right angles to the surface of the coated layer, exposing the ceramic slurry to an air blow from a plurality of nozzles, drying the ceramic slurry by tilting the support, applying the ceramic slurry by means of a doctor blade or an applicator which has a regular surface unevenness, and applying the ceramic slurry by a screen printing technique which involves a rotary screen. By the use of ceramic greenware sheets with electrodes embedded therein the surface of each of which are effectively smoothed, it is possible to manufacture laminated-ceramic electronic components in which a high degree of lamination has been achieved with thin layers.
Thus, according to the invention, because the electrodes are dried, a possibility that the ceramic greenware sheets are eroded by a solvent included in the electrode ink resulting in swelling and short circuit, is reduced. Moreover, steps generated due to the electrodes are reduced by embedding the electrodes in the ceramic greenware sheets even when the dried electrodes are applied to the manufacture of multi-layered laminated-ceramic capacitors.
A gravure printing technique which uses no impression cylinder can be also adopted when embedding the electrodes in the ceramic greenware sheets. In this case, a ceramic slurry can be applied without damaging the electrodes which are formed on the surface of a support.